
The enthalpy of the material is computed as the sum of the sensible enthalpy, , and the latent heat, :
where
(24.22) 
and  =  reference enthalpy  
=  reference temperature  
=  specific heat at constant pressure 
The liquid fraction, , can be defined as
Equation 24.23 is referred to as the lever rule .
The latent heat content can now be written in terms of the latent heat of the material, :
The latent heat content can vary between zero (for a solid) and (for a liquid).
In the case of multicomponent solidification with species segregation; i.e., solidification or melting with species transport, the solidus and liquidus temperatures are computed instead of specified (Equations 24.25 and 24.26).
For solidification/melting problems, the energy equation is written as
where  =  enthalpy (see Equation 24.21)  
=  density  
=  fluid velocity  
=  source term 
The solution for temperature is essentially an iteration between the energy equation (Equation 24.27) and the liquid fraction equation (Equation 24.23). Directly using Equation 24.23 to update the liquid fraction usually results in poor convergence of the energy equation. In FLUENT, the method suggested by Voller and Swaminathan [ 386] is used to update the liquid fraction. For pure metals, where and are equal, a method based on specific heat, given by Voller and Prakash [ 385], is used instead.